Foreign Keys are often a mystery to new DBAs in the MySQL world. Hopefully this blog will clear some of this up.

In this example, we will have a table for employee data and a table for the data on offices. First we need the two tables.CREATE TABLE employee (
-> e_id INT NOT NULL,
-> name CHAR(20),
-> PRIMARY KEY (e_id)
-> );

Those who do not use Foreign Keys will not be familiar with the last four lines of the building table. The trick is that there are two e_id columns, one in each table. In the employee table is it simply the employee identification number. However in building table, it is declared to be a foreign key to the employee table using the e_id column. The CASCADE lines are telling MySQL that any UPDATEs or DELETEs on the e_id column in employee table will also be made on the corresponding row(s) in the building table.

But we have three employees and only two lines of output? What happened? Well, what happened is that the query wanted the matches from both tables. To get all the rows from the first table and any matches from the second table, use a LEFT JOIN.mysql> SELECT * FROM employee LEFT JOIN building ON (employee.e_id=building.e_id);
+------+-------+------------+---------------+------+
| e_id | name | office_nbr | description | e_id |
+------+-------+------------+---------------+------+
| 10 | Larry | 100 | Corner Office | 10 |
| 40 | Moe | 101 | Lobby | 40 |
| 20 | Shemp | NULL | NULL | NULL |
+------+-------+------------+---------------+------+
3 rows in set (0.00 sec)

Much better.

A big benefit of using foreign keys is that bad values get a lot harder to insert into the database. Try to add an office for a non-existent employ number 77.mysql> INSERT INTO building VALUES (120,'Cubicle',77);ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`test`.`building`, CONSTRAINT `building_ibfk_1` FOREIGN KEY (`e_id`) REFERENCES `employee` (`e_id`) ON DELETE CASCADE ON UPDATE CASCADE)
mysql>

Now back to all that CASCADE stuff. Remove any of the employees from the employee table and the corresponding building entry will be removed.mysql> DELETE FROM employee WHERE e_id=40;
Query OK, 1 row affected (0.08 sec)

There are many MySQL DBAs who do not use Foreign Keys for various reasons but they can be very handy. I find them useful in one to many relationships where I do not want to have to purge or change the many directly in a query.